1. Field of the Invention
The present invention relates to a technique used for adjusting focus of an imaging unit to which an object image is input via a focus lens.
2. Description of the Related Art
Conventionally, electronic still cameras use autofocus (AF) methods in moving a position of a focus lens and focusing on an object. According to the AF methods, focusing operations are automatically performed using a luminance signal acquired from an image sensor such as a charge-coupled device (CCD). An automatic focusing apparatus using the AF method generally acquires an in-focus point by using an AF evaluation value. The AF evaluation value is obtained by integrating a high-frequency component of a luminance signal in a focusing area set in each image plane. According to the AF evaluation value, a focus lens position with the highest contrast is detected. Then, the in-focus point is acquired from the detected lens position.
The hill-climbing method (hereinafter referred to as continuous AF) is known among the AF methods. According to the continuous AF, the focus lens moves in the direction where the AF evaluation value increases so that a position with the maximum AF evaluation value is detected. The detected point is determined to be an in-focus position. If the focus lens moves slowly in the direction where the AF evaluation value increases according to this method, the object can be focused without reducing the quality of a live image. However, if the object moves a great deal toward or away from the camera, accuracy of the focus tracking is decreased.
The focus tracking of an object moving a great deal toward or away from the camera can be improved if the drive speed of the focus lens is increased. As an AF method for improving focus tracking of such an object, a method that tracks an in-focus point by continuously and rapidly driving the focus lens in a range based on the in-focus points detected in the past has been developed. This method is referred to as servo AF in the following description. However, in improving the focus tracking with respect to an object which is moving a great deal toward or away from the camera, the focus change per unit time is increased, and accordingly, the quality of a live image becomes poor.
Japanese Patent Application Laid-Open No. 2007-282119 discusses an electronic camera which detects a state of a main object (person) and controls the image capturing operation according to the detected state. Japanese Patent Application Laid-Open No. 2006-157428 discusses a photographing apparatus which predicts motion of an object and motion of the photographing apparatus. Various image capturing conditions including shutter speed, aperture value, sensitivity, and flash condition are set according to the predicted motion of the object.
According to the technique discussed in Japanese Patent Application Laid-Open No. 2007-282119, the state of the object being the main person is detected, and then the image capturing operation is controlled according to the detected state. However, since the AF control is not controlled according to the motion of the detected object, focus on a moving object is not tracked as desired. Further, in photographing an object which is not moving, the quality of a live image is decreased due to a change in focus.
According to the technique discussed in Japanese Patent Application Laid-Open No. 2006-157428, motion of an object and motion of the photographing apparatus are predicted and various image capturing conditions including shutter speed, aperture value, sensitivity, and flash condition are set according to the predicted motion of the object. However, since the AF control is not changed according to the motion of the detected object, focus on a moving object is not tracked as desired. Further, in photographing an object which is not moving, the quality of a live image is decreased due to a change in focus.